The present invention relates to a vehicle power steering system, and more specifically to a hydraulic vehicle power steering system in which the resistance to actuation of a power steering control valve increases with increasing vehicle speed.
A known rotary power steering control valve shown in U.S. Pat. No. 5,046,574 has an inner valve member which is coaxial with and rotatable relative to an outer valve member or sleeve. To effect actuation of the power steering motor to turn steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member. A fluid pressure reaction chamber is provided to regulate the torque required to rotate the inner valve member relative to the outer valve member. The fluid pressure in the reaction chamber increases as vehicle speed increases to increase the resistance felt by an operator of the vehicle to rotation of the inner valve member relative to the outer valve member.
In U.S. Pat. No. 5,046,574 the fluid pressure in the reaction chamber is independent of vehicle steering activity. Hence, a power steering pump has to act continuously against an increasing pressure as vehicle speed increases. Thus, there is a loss of horsepower and heat generated due to the pump acting against the relatively high pressure at high vehicle speeds when there is no steering activity.